The government has rights in this invention under a grant from the National Institutes of Health, NIH-5-R01-CA37235.
This invention relates to the calibration of polarographic sensors.
Polarographic sensors are used to determine the concentration or the thermodynamic chemical potential of many chemical species. In particular, the determination of the chemical potential of oxygen, or oxygen tension, is useful in the laboratory, in biomedicine, and in industrial process and environmental monitoring. See, Polarography, D. R. Crow et al. (1968), Methuen and Co. London and Polarographic Oxygen Sensors: Its Theory of Operation and Its Application in Biology, Medicine and Technology, I. Fatt (1982) Krieger, Malabar, Fla. Typically, a polarographic sensor is polarized in some fashion with respect to a non-polarizable reference electrode, often a silver/silver chloride electrode. The sensor is frequently fabricated from a noble metal, for example gold or platinum.
The polarographic sensor is placed in a medium, for example, living tissue, that may contain a chemical species whose concentration or thermodynamic chemical potential is to be measured. The species to be determined is oxidized or reduced on the sensor. The flow of current to or from the sensor, used to complete the electrochemical oxidation or reduction reaction, is indicative of the concentration of the species being oxidized or reduced. This electrochemical current is also a function of the thermophysical properties of the species in the medium, especially its solubility and diffusivity. See, "The Measurement of Oxygen Diffusivity and Concentration by Chronoamperometry Using Microelectrodes," Journal of Electroanalytical Chemistry, C. P. Winlove et al., Vol. 170, 1984, pp. 293-304. Because the electrochemical current can be a function of the thermophysical properties of the species in the medium, sensors are often coated with materials of known thermophysical properties so as to minimize the effect of variation of properties, such as solubility and diffusivity, on the measurement. The thermophysical properties of the sensor coating most often become known through calibration of the coated probe in media with known concentrations or chemical potentials of the species being measured.
Sensor coatings may consist of membrane encased electrolytic solutions covering the sensor and reference electrode, or of permeable solids and/or gels covering one or both sensors. See, "Some Considerations of the Steady State and Transient Behavior of Membrane-Covered Dissolved Oxygen Detectors," Journal of Electroanalytical Chemistry, J. M. Hale et al., Vol. 107, 1980, pp. 293-304; "Preliminary Study of on-line Computation of Cardiac Output Using Indwelling Oxygen Catheters in Man," J. L. Hoffer et al., Federation Proceedings, Vol. 31, 1972, p. 786; and "Absolute P02 Measurements with Pt-Electrodes Applying Polarizing Voltage Pulsing," K. Kunze et al., Advances in Experimental Medicine and Biology, Vol. 37A, 1973, pp. 35-43.
The sensor coatings render the measurement one of thermodynamic chemical potential of the species as opposed to concentration of the species in the medium. The dependence of the measurement on the thermophysical properties of the species in the medium is largely removed, but as the sensor is not directly contacting the medium, an effect on the species in the medium is not registered (measured) by the sensor for some time period after the effect occurs. The time period is determined by the coating thickness and thermophysical properties of the coating material. The time lag is also characterized by large transient current upon initial polarization of the sensor while excess amounts of the species in the coating are consumed. See, Polarography, 2nd Edition Interscience, I. M. Kolthoff et al., (1952) New York. This time lag is a characterizable property of the electrode.
In general, however, it is the long time (steady state) behavior of the current that has been used to determine the chemical potential of the species. In order to minimize the dependence of the measurement on the thermophysical properties of the species in the medium, coatings have been chosen which are relatively thick, and/or ones in which the species are relatively insoluble and/or slowly diffusive. Such coatings result in a slowly responsive sensor with a relatively low level signal.